1. Field of the Invention
The present invention relates to a surface acoustic wave device that is used, for example, as a resonator or a band-pass filter. More particularly, the present invention relates to a surface acoustic wave device that has a structure in which an IDT electrode and a silicon oxide film are disposed on a LiNbO3 substrate and that utilizes an SH wave.
2. Description of the Related Art
A band-pass filter used in an RF stage of a cellular phone, or the like, is required to have a wide band and an outstanding temperature characteristic. Thus, the existing art employs a surface acoustic wave device in which an IDT electrode is disposed on a piezoelectric substrate made of a Y-rotated X-propagating LiTaO3 substrate or a Y-rotated X propagating LiNbO3 substrate and a silicon oxide film is disposed to cover the IDT electrode. The piezoelectric substrate of this type has a negative frequency-temperature coefficient. Thus, in order to improve the temperature characteristic, a silicon oxide film having a positive frequency-temperature characteristic is disposed so as to cover the IDT electrode.
However, when the IDT electrode is made of Al or an alloy that mostly contains Al, which are generally used, the IDT electrode has not been able to have a sufficient reflection coefficient. Thus, there has been a problem in which a ripple occurs in the resonance characteristic.
WO 2005-034347 describes a surface acoustic wave device that solves the above problem, a description of which follows. Here, an IDT electrode made of a metal having a density that is greater than that of Al is disposed on a piezoelectric substrate made of LiNbO3 having an electromechanical coefficient K2 of greater than or equal to 0.025, a first silicon oxide film is disposed in the area, other than the area in which the IDT electrode is disposed, so as to have a thickness equal to the electrode, and a second silicon oxide film is laminated so as to cover the electrode and the first silicon oxide film.
In the surface acoustic wave device described in WO 2005-034347, the density of the IDT electrode is greater than or equal to one and half times the density of the first silicon oxide film. Thus, the reflection coefficient of the IDT electrode is sufficiently increased to thereby make it possible to suppress a ripple that occurs in the resonance characteristic.
In addition, in WO 2005-034347, a Rayleigh wave is utilized and Au, Cu, or the like, is shown as the electrode material. The structure in which the electrode made of Cu has a thickness of 0.0058λ to 0.11λ is described. In this case, the LiNbO3 substrate having Euler angles of (0°±5°, 62° to 167°, 0°±10°), preferably, (0°±5°, 88° to 117°, 0°±10°), is described, and the thickness of the second silicon oxide film ranges from 0.15λ to 0.4λ where λ is a wavelength of a surface wave.
In the surface acoustic wave device described in WO 2005-034347, when θ of the Euler angles of the LiNbO3 substrate, the thickness of the electrode made of Cu and the thickness of the second silicon oxide film are set to fall within the above specific range, as described above, it is possible to increase the electromechanical coefficient KR2 when a Rayleigh wave is utilized, and it is possible to reduce the electromechanical coefficient of a mode that becomes a spurious response.
On the other hand, a duty of an IDT electrode is possibly reduced in the surface acoustic wave device in order to enhance power withstanding performance. The duty is a ratio of a size of an electrode finger in width direction to a sum of a size of the electrode finger in the width direction and a gap between the adjacent electrode fingers. In the surface acoustic wave device described in WO 2005-034347, the duty of the IDT electrode is not considered.
In the surface acoustic wave device described in WO 2005-034347, when the duty is reduced to a value less than 0.5 in order to enhance power withstanding performance, the power withstanding performance is enhanced, however, a large spurious response sometimes occurs. That is, even when the range of the thickness of an electrode film and the range of θ of Euler angles are selected as described in WO 2005-034347, if the duty is reduced in order to enhance power withstanding performance, a large spurious response sometimes occurs.
In addition, depending on conditions on which the surface acoustic wave device is manufactured, a spurious response sometimes occurs due to change in size of each electrode finger in the width direction by the conditions of manufacturing, or the like.